1. Field of the Invention
This invention relates to coherent radiation devices and more particularly to optically pumped lasers, optical amplifiers, optical beam cleanup devices, optical beam combiners, and frequency up-conversion devices having input and output beams ranging in wavelengths from centimeters to nanometers.
2. Description of the Prior Art
MASERS, LASERS, AND OPTICAL AMPLIFIERS
The principal requirement for producing a laser or optical amplifier is that the laser or amplifier medium be inverted. This means that in the transition of interest there is a greater population of atoms or molecules in the upper quantum state than in the lower quantum state. Such media display "negative" absorption or gain. A variety of methods are used to provide population inversions. In 1954 the first microwave laser (maser) was developed (J. P. Gordon, H. J. Zeiger, and C. H. Townes, "The Maser-A Type of Microwave Amplifier, Frequency Standard, and Spectrometer," Phys. Rev. 99, 1264-1274, August, 1955) in which an inverted population was achieved in ammonia gas by a spatial separation of energy states. In 1958 the optical maser, or laser, was proposed (A. L. Schawlow and C. H. Townes, "Infrared and Optical Masers," Phys. Rev., 112, 1940-1949, December, 1958), and in 1960 the first laser was in operation (T. H. Maiman, "Stimulated Optical Radiation in Ruby Masers," Nature, 187, 493-494, August, 1960).
Three features are common to all lasers and maser (R. H. Pantell and H. E. Puthoff, Fundamentals of Quantum Electronics, John Wiley and Sons, Inc., New York (1969) pp. 101-103):
1. An excitation mechanism. This mechanism may be electromagnetic radiation, charged particle beams or currents, chemical reactions, etc. The purpose of the mechanism is to invert the gain medium so that the inversion energy may be extracted by a coherent electromagnetic beam.
2. An active medium. This medium may be a gas, liquid, solid, or plasma that sustains the inverted population.
3. A circuit. Electromagnetic radiation from the active medium is coupled to the external environment by means of a circuit. At infrared or optical frequencies the circuit is usually an interferometer resonator, and for masers a cavity resonator is used.
Optical amplifiers require features (1) and (2), but not feature (3). In place of the circuit, a small input beam is used which is amplified by its passage through the inverted medium thus extracting the stored energy from the medium.
FIG. 1 displays a typical population inversion scheme. The pump quantum drives the system from level 1 to level 4. The system (atom molecule, etc.) then decays due to any number of mechanisms (spontaneous emission, collisional decay, etc.) from level 4 to level 3. The decay rate into level 3 (the upper level of the inverted level pair) is greater than the decay rate out of that level. This leads to a population buildup in level 3 which, when it exceeds the population in level 2 (the lower level of the inverted level pair), allows for optical gain through stimulated emission. A number of other pumping schemes have been proposed and demonstrated. All have one essential feature in common. In current laser, maser, and optical amplifier pumping techniques the pump quantum (optical, chemical, mechanical, etc.) which is used to excite the gain medium contains more energy than the lasing or amplified quantum. This, in effect, makes optically pumped lasers, masers, and optical amplifiers frequency down-conversion devices.
OPTICAL BEAM CLEANUP DEVICES
In order to accurately and efficiently transfer coherent electromagnetic energy from its generating source to a desired location it is necessary to have a well defined (usually flat) phase across the beam. Optical beams whose phase fronts are flat are said to have good beam quality. There are many processes in laser, maser, and optical amplifier devices which tend to distort this phase and thus degrade the beam quality of these devices. A number of mechanical and nonlinear optical techniques have been proposed and demonstrated which provide some measure of correction. Two basic procedures are currently used:
1. Phase front cleanup via mechanical devices such as deformable mirrors and nonlinear optical techniques such as optical phase conjugation (see e.g. R. A. Fisher (Ed.), Optical Phase Conjugation, Academic Press, New York (1983) pp. 1-22). These devices may be part of or external to the beam generation device. Deformable mirrors are limited to correction of phase imperfections whose scale size is no smaller than the spacing between mirror actuators. Nonlinear optical phase conjugation requires high intensities (usually MW/cm.sup.2 or higher).
2. Energy transfer from a beam with poor beam quality to another beam which maintains good beam quality. Typically a nonlinear optical process is used which requires high intensities (often MW/cm.sup.2 or higher). At such intensities other parasitic nonlinear optical processes often occur. Stimulated Raman scattering has been studied extensively for this purpose, (see e.g. A. Penzkofer, A. Laubereau, and W. Kaiser, "High Intensity Raman Interactions," Prog. Quant. Electr., 6, 55-140, 1980 pp. 56-57) as have other parametric wave mixing processes (see e.g. J. F. Reintjes, Nonlinear Optical Parametric Processes in Liquids and Gases, Academic Press, Inc. New York, 1984 ppg. 1-30) such as multiwave mixing and optical parametric amplification. These processes, in general, require accurate phase matching of input beams which in turn limits the applicability of these processes for many applications.
FREQUENCY UP-CONVERSION DEVICES
A number of nonlinear optical techniques have been exploited to up-convert coherent electromagnetic beams. Typcially this involves phase matched, high intensity (MW/cm.sup.2) parametric wave mixing such as harmonic generation (see e.g. Y. R. Shen, Principles of Nonlinear Optics, Academic Press, New York, 1984 pp. 86-107) or anti-Stokes Raman generation (see e.g. A. Penzkofer, A. Laubereau, and W. Kaiser, "High Intensity Raman Interactions," Prog. Quant. Electr., 6, 55-140, 1980 pp. 56-57).
OPTICAL BEAM COMBINERS
It is not possible to scale lasers to arbitrary size and power. As such, a number of devices have been proposed and/or demonstrated which would allow a number of laser beams to be combined into one more powerful beam. The essential requirement is that the individual laser beams be combined coherently, i.e., that the N beams are combined in such a fashion that each is in phase with all N-1 other beams. In this case the intensity of a combination of N beams is N.sup.2 times the intensity of each combined beam individually. A number of methods have been proposed and/or demonstrated which provide beam combination under certain conditions:
1. Phased arrays. These devices generally adjust the optical path length that beams from different lasers must travel so that the wave fronts of the beams from each laser matches those of the other lasers. This requires path control of much better than a wavelength (typcially 300 nanometers to tens of microns), and usualy on the order of one tenth to one fiftieth of a wavelength. Difficulties arise in large laser systems where there are typically significant mechanical vibrations and often multiaxial mode outputs which prevent path matching of all axial modes from the combined lasers.
2. Coupled resonators, injection locked resonators, master-oscillator-power-amplifiers (MOPA) configurations. By feeding power from one laser to one or more other lasers it is possible under certain conditions to lock the phases of a number of lasers together (M. Sargent III, M. O. Scully, W. E. Lamb Jr., Laser Physics, Addision-Wesley Reading, Mass., 1974, pp. 45-54, M. B. Spencer and W. E. Lamb, Jr., Phys. Rev. A 5, 884-892, (1972). In general the difficulties encountered in (1), directly above, apply here also.
3. Nonlinear Optical beam combining. These techniques include stimulated Brillioun scattering (see e.g. Y. R. Shen, Principles of Nonlinear Optics, John Wiley, New York, 1984, pp. 187-192), simulated Raman scattering, optical phase conjugation, and other parametric wave mixing techniques. As above, high intensity beams and phase matching conditions are required.